The eutrophication in lakes has been known as one of the most significant problems in water quality management. The problems due to the eutrophication are increases in algal biomass, oxygen depletion in lakes, teate and odour, and clogging of sand filter beds in a domestic water supply process. In this paper, a simple water quality model was developed and applied for the Andong dam with respect to the control of eutrophication. As a result of micro-computer based modeling approach, the BOX model demonstrated good performances in terms of the accuracy of prediction in water quality in a lake.